@NullAndEmptySource in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a JUnit test using @NullAndEmptySource with String parameter

Consider this JUnit 5 parameterized test using @NullAndEmptySource. What will be the total number of test invocations executed?

JUnit

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.NullAndEmptySource;

class SampleTest {
    @ParameterizedTest
    @NullAndEmptySource
    void testWithNullAndEmpty(String input) {
        System.out.println(input == null ? "null" : input);
    }
}

A 0

B 1

C 2

D 3

💡 Hint

Think about how many inputs @NullAndEmptySource provides to the test method.

The @NullAndEmptySource annotation provides exactly two inputs to the test method: null and an empty string "". So the test runs twice.

❓ assertion

intermediate

Correct assertion for @NullAndEmptySource inputs

In a parameterized test using @NullAndEmptySource for a String parameter, which assertion correctly verifies that the input is either null or empty?

JUnit

import static org.junit.jupiter.api.Assertions.*;

@ParameterizedTest
@NullAndEmptySource
void testInput(String input) {
    // Which assertion is correct here?
}

A assertFalse(input == null || input.isEmpty());

B assertNotNull(input);

C assertEquals("", input);

D assertTrue(input == null || input.isEmpty());

💡 Hint

Remember @NullAndEmptySource provides null and empty string inputs.

The inputs are either null or empty string "". The assertion must check for both conditions combined with OR.

🔧 Debug

advanced

Why does this @NullAndEmptySource test fail with a NullPointerException?

Given this test code, why does it throw a NullPointerException during execution?

JUnit

import static org.junit.jupiter.api.Assertions.*;

@ParameterizedTest
@NullAndEmptySource
void testLength(String input) {
    assertTrue(input.length() == 0);
}

A Because empty string has no length property.

B Because input can be null, calling length() on null causes NullPointerException.

C Because @NullAndEmptySource does not provide any inputs.

D Because assertTrue is used incorrectly.

💡 Hint

Think about what happens when input is null and you call a method on it.

@NullAndEmptySource provides null and empty string. Calling input.length() when input is null causes a NullPointerException.

🧠 Conceptual

advanced

Behavior of @NullAndEmptySource combined with @ValueSource

What happens when you combine @NullAndEmptySource with @ValueSource(strings = {"test"}) in a parameterized test method with a String parameter?

JUnit

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.NullAndEmptySource;
import org.junit.jupiter.params.provider.ValueSource;

@ParameterizedTest
@NullAndEmptySource
@ValueSource(strings = {"test"})
void testCombined(String input) {
    System.out.println(input);
}

A The test runs three times with inputs: null, empty string, and "test".

B The test runs only once with input "test".

C The test runs twice with inputs: null and empty string only.

D The test fails to run due to conflicting annotations.

💡 Hint

Think about how multiple sources combine in JUnit parameterized tests.

JUnit combines all sources, so the test runs once for each input from both annotations: null, empty string, and "test".

❓ framework

expert

How does JUnit 5 internally handle @NullAndEmptySource for parameterized tests?

Which statement best describes how JUnit 5 processes the @NullAndEmptySource annotation when running a parameterized test?

A JUnit 5 provides two arguments to the test method: null and an empty string, by internally creating these values as test inputs.

B JUnit 5 reads @NullAndEmptySource and skips the test method if the parameter is not nullable.

C JUnit 5 requires the user to manually provide null and empty string values; @NullAndEmptySource is just a marker.

D JUnit 5 converts @NullAndEmptySource into a single empty string input only.

💡 Hint

Consider how parameterized tests get their input values from annotations.

@NullAndEmptySource is a built-in source that automatically supplies two inputs: null and empty string "" to the test method.